"For starters..." (7)
'for starters' is the definition.
(I've seen this in another clue)
This is all the clue.
(Another definition for firstly that I've seen is " Primarily".)
(Other definitions for first that I've seen before include "best" , "The one that precedes all others, in time say" , "higher than the others" , "Leading" , "Most important" .)